1. Field of the Invention
The present invention relates to a lens drive mechanism for moving a lens group linearly along an optical axis.
2. Description of the Related Art
Conventional lens drive mechanisms mainly for moving a focusing lens group (movable lens group) linearly along an optical axis are generally provided with a lens support frame which supports the focusing lens group, a rotatable lead screw (feed screw shaft) rotatable on its axis and parallel to the optical axis, a motor (stepper motor) for rotating the rotatable lead screw, a guide shaft parallel to the optical axis, and a nut member (driven member) which is supported by the rotatable lead screw and the guide shaft to be linearly moved together with the lens support frame in the optical axis direction by a rotation of the rotatable lead screw. This type of conventional lens drive mechanism is disclosed in Japanese Unexamined Patent Publication No. 2003-222932.
As an example of such a type of lens drive mechanism, a lens drive mechanism in which the nut member is provided with a screw engaging portion which is screw-engaged with the rotatable lead screw and an engaging groove in which the guide shaft is engaged to prevent the nut member from rotating about the rotatable lead screw by the engagement of the guide shaft with the engaging groove is known in the art. In this type of lens drive mechanism, a rotation of the rotatable lead screw causes the nut member to move linearly together with the lens support frame along the guide shaft (and the rotatable lead screw).
The nut member which serves as an element of the above described type of lens drive mechanism is generally made of metal. However, in the case where the nut member is made of metal, the frictional resistance between the nut member and the guide shaft becomes great if the guide shaft and the engaging groove of the nut member are in contact with each other at all times, thus making the linear movement of the nut member is not smooth. To overcome this problem, it is common to form the engaging groove so as to have a large inner diameter to create a clearance between the guide shaft and the engaging groove.
However, if a substantial clearance exits between the guide shaft and the engaging groove, the nut member (the engaging groove thereof) and the guide shaft hit against each other to make objectionable noise during linear movement of the nut member.